In general, conventional solar cells have the structure shown in FIG. 1. In FIG. 1, a p-type semiconductor substrate 1 is typically a plate of poly- or monocrystalline silicon dimensioned 100 to 150 mm squares by 0.1 to 0.3 mm thick which is doped with a p-type impurity such as boron. On the substrate 1, an n-type diffusion layer 2 is formed by doping it with an n-type impurity such as phosphorus, and an antireflection film 3 of silicon nitride (SiN) or the like is formed. On the back surface, a back electrode 6 and a back-surface field (BSF) layer 4 are formed at the same time by printing a conductive aluminum paste thereto by screen printing technique, drying and firing. On the front surface, a collector electrode (or front electrode) 5 is formed by printing a conductive silver paste thereto, drying and firing. A solar cell is manufactured in this way. It is noted that the front surface refers to one surface of a substrate serving as the light-receiving side of a solar cell and the back surface refers to another surface of the substrate opposite to the light-receiving side.
When the collector electrode is formed by the screen printing technique, a single printing step is limited to a thickness of 30 μm at maximum relative to a line width of 100 μm, with a substantial thickness variation. The electrode resistance thus increases, becoming a deleterious factor against an improvement in conversion efficiency.
For the purpose of reducing the resistance value of a collector electrode, a method of forming a collector electrode by repeating screen printing plural times and using a mesh screen of different pattern in each of plural screen printing steps is under study (see, for example, JP-A 2010-103572). The solar cell manufactured by such a method generally has the structure shown in FIG. 2. As a result of overlay printing, the front electrode has an increased height. In FIG. 2, the front electrode includes a second layer 7.
However, if the mesh screens have different patterns, with which the degree of spreading under a printing pressure applied differs, the precision of printing is adversely affected. It is difficult to overlay collector electrode layers accurately. A misregistration occurs between collector electrode layers, resulting in a solar cell with low conversion efficiency or an appearance problem due to local thickening of collector electrode.